This invention relates to aircraft, and more particularly to a new concept of a counter-rotating proprotor designed for wingtip mounting on tiltwing aircraft.
Tiltwing aircraft are designed to tilt their wing from a horizontal position in cruise to a vertical position in hover. They operate like a helicopter with the wing in the vertical position. For high speed forward flight, the wing tilts to the horizontal position and operates in a manner similar to that of a standard turboprop airplane. To successfully convert the aircraft from hover to cruise (conversion) or from cruise to hover (reconversion) the wing must operate over a wide range of angles of attack without stalling. The propulsors (usually propellers or a hybrid propeller/rotor combination called a proprotor, which provides the attributes of a propeller when in the level flight (cruise) position and the attributes of a helicopter rotor when in the vertical (hover) position) have a strong impact on the wing's angle of attack range since they (1) increase the velocity over the wing (this is beneficial), (2) deflect the flow over the wing (this too is beneficial), and (3) in single rotation propulsors impart a swirl component to the flow over the wing. This third feature may be either beneficial or detrimental, depending upon the direction of the swirl component. In order to reduce the power requirements of tiltwing aircraft, thereby making them more efficient and more attractive with respect to conventional single-mission type aircraft, it is desirable to move the proprotors outboard to the wingtip, so that larger diameter (lower disk loading) proprotors may be used. This has a negative impact on the aircraft's conversion envelope (the combination of altitude, descent rate, and power setting) however, because reducing the disk loading also reduces the velocity increase over the wing and the deflection angle of the proprotor slipstream.
For low disk loading, single-rotation, wingtip-mounted proprotors, the swirl imparted to the flow has a significant impact on a tiltwing aircraft's conversion envelope and cruise performance. If the proprotor is turned in a direction opposite to the wingtip vortex, a wing efficiency benefit is realized. However, during conversion, turning the proprotor in this direction causes a swirl component that increases the wing's angle of attack, causing the wing to stall at a lower incidence angle. This reduces the conversion envelope. Contrarily, if the proprotor is turned in the same direction as the wingtip vortex, the conversion envelope is increased but the wing efficiency is decreased. Thus, the designer is faced with a paradox of either decreasing the conversion envelope or decreasing the wing efficiency.
What is needed, therefore, is a new approach which does not force a tradeoff between the size of the conversion envelope and the wing efficiency, while permitting an increased cruise (or propulsor) efficiency.